Masonic lodge regalia is traditionally trimmed with blue. For
instance, the white cloth aprons members wear during meetings are
trimmed in blue, and the officers insignia collars are usually blue.

In some grand lodges, a particular shade of blue is specified: from
as light as light sky blue, through deep royal blue, sometimes as dark
as navy. Others let their individual lodges choose any blue they like.

No one really knows why blue was chosen. Probably it's supposed to
signify sky and thereby "heaven" and "celestial" things. I have read a
number of fanciful (mostly b.s.) essays on the subject but nothing with
any real evidence.

As late as the middle of this century, I think blue lodge
was considered an improper slang usage among learned Masons. The writer
Carl Claudy certainly didn't like it.

"Blue lodge" refers to the fact that other appendant orders or
side-bodies use other colors. For example: in the Royal Arch, the
regalia are trimmed with red; in the Cryptic Degrees they use purple,
etc. So maybe the term was a "back-formation," and wasn't invented to
describe regular craft lodges until after all those other bodies came
along.

In order to say "not any of those other bodies, but the regular
three-degree Master Mason's craft lodge" people apparently began using
the term "blue lodge" -- indicating "not purple, not red, not white, not
purple nor black nor gold" or whatever. The more correct terms were
Craft lodge or Master Mason's Lodge. Nowadays
blue lodge has caught on as is in wide use, and is no longer
considered incorrect by most, or maybe all.

Interestingly enough, I have recently even heard the term blue
lodge mis-used. When you're raised initiated
(receive the 1st or 'Entered Apprentice' Degree) in a particular lodge,
and made a member there that lodge is termed your
mother lodge. You might join (affiliate with) other lodges,
and even leave (demit from) this original lodge, but the lodge where you
were raised will always be your mother lodge. It's a
very precise term with a very exact meaning.

Recently, in a lodge where we are both members, a brother asked me
"what is your blue lodge?" Of course, I suppose that every Craft lodge
where I am affiliated could also count as "my blue lodge", so the
question made no sense at all. He was trying to ask "what is your
mother lodge?"

So maybe the term blue lodge will evolve some more until
sometimes it will mean mother lodge and sometimes it won't.
Then I don't know what we'll use for blue lodges that aren't one's
mother lodge.